The theory of evolution by natural selection was developed by Charles Darwin in the 19th century. His groundbreaking work was primarily published in his book "On the Origin of Species" in 1859. Darwin proposed that species evolve over time through a process where individuals with advantageous traits are more likely to survive, reproduce, and pass these traits to their offspring. This theory fundamentally changed the understanding of biology and the diversity of life on Earth. Darwin's ideas were influenced by his observations during the voyage of the HMS Beagle and the work of earlier naturalists like Alfred Russel Wallace.